The present invention pertains generally to devices and methods for imaging the internal features of an object. More particularly, the present invention pertains to improved devices and methods for imaging the internal features of an object while using a conventional broadband X-ray source. The present invention is particularly, but not exclusively, useful for producing an enhanced-contrast image of the internal features of an object by using filtered X-ray radiation.
The ability to image the internal features of an object is important in many applications. Two examples include medical diagnosis and the non-destructive testing of structural components to detect configuration or discover internal flaws. In all applications, it is desirable to produce an image having high contrast and spatial resolution. Radiation within the X-ray spectrum is often used to image internal features because of the ability of X-ray radiation to penetrate matter and because different matter absorbs X-ray radiation at different rates. Typically, a conventional X-ray source produces an emission of X-ray radiation having a broad range of energies. In conventional imaging applications, these X-rays are directed through the object for subsequent capture by a film or detector. The imaging films and detectors used are responsive to the intensity of the radiation received, and thus are able to produce an image of the internal features of the object when those internal features have differing absorption characteristics.
To enhance the contrast and spatial resolution of an image, contrast agents are often used. Specifically, these agents include chemical elements that have absorption rates that are significantly different than the constituents of the object to be imaged. For example, iodine can be administered within the body as a contrast agent. Once administered, the iodine is selectively absorbed by certain tissues or is present within the blood vessels. Subsequently, when an X-ray image is formed, areas of the body with large amounts of iodine will absorb relatively greater amounts of X-ray radiation than areas of the body without iodine. Thus, contrast agents can be used with good efficacy to increase both the contrast and the spatial resolution of the image.
FIG. 1 shows the variation of absorption coefficient with radiation energy for a typical chemical element. In simple terms, the absorption coefficient decreases as the energy increases until an energy is reached that is sufficient to knock a K-shell electron from it""s orbit. At this energy, EK-EDGE, the value of the absorption coefficient jumps. For purposes of the present disclosure, the term KEDGE is used to denote the energy at which the jump in absorption coefficient occurs. Continuing with FIG. 1, it can be seen that further increases in energy again result in a gradual decrease in absorption coefficient.
The present invention recognizes that the variation in absorption coefficient near KEDGE can be utilized to increase image contrast. Specifically, the present invention recognizes that image contrast can be increased by first introducing a contrast agent having a known KEDGE into the object. Next, a first image can be formed using monochromatic radiation having an energy just less than KEDGE for the contrast agent (such as radiation having an energy, E1, in FIG. 1) and a second image formed using monochromatic radiation having an energy just greater than KEDGE for the contrast agent (such as radiation having an energy, E2, in FIG. 1). When this is done, the resulting two images can be compared (subtracted) to produce a high contrast image of the internal features of the object.
One way to produce the monochromatic X-ray radiation needed to conduct the above described subtraction method is to use a crystal monochromator. Unfortunately, when a crystal monochromator is used in conjunction with a conventional X-ray source, the intensity of the resulting monochromatic radiation is so reduced that the radiation is insufficient for almost all practical imaging uses. One way to produce monochromatic radiation at a suitable intensity for imaging is to pass the high intensity radiation from a synchrotron source through a double crystal monochromator. As can be expected, producing radiation with a synchrotron source is very expensive. Further, the beam produced by the synchrotron source/double crystal monochromator is fixed in direction and, consequently, cannot be rapidly moved as required in a typical tomographic scan.
In light of the above, it is an object of the present invention to provide devices and methods suitable for the purposes of producing a digital image signal that is substantially equivalent to an image signal obtained by passing quasi-monochromatic radiation (i.e. radiation having a narrow energy band) through an object. It is another object of the present invention to provide devices and methods for producing images of the internal features of an object having enhanced contrast and high spatial resolution. It is yet another object of the present invention to provide devices and methods for use in conjunction with standard computer tomography or angiography equipment to enhance the contrast and increase the spatial resolution of the images produced. It is yet another object of the present invention to provide devices and methods for enhancing image contrast and increasing spatial resolution that can be used with a variety of different contrast agents. Yet another object of the present invention is to provide an X-ray Filter System For Medical Imaging Contrast Enhancement which is easy to use, relatively simple to manufacture, and comparatively cost effective.
The present invention is directed to a system for creating an image of the internal features of an object. Specifically, the present invention is directed at imaging an object that contains a contrast agent. For the present invention, the system includes an X-ray source configured to produce a spectrum of X-ray radiation. An optional collimator may be provided to collimate the radiation emitted from the X-ray source into one or more beams. As such, each beam emanates from the X-ray source in a slightly different direction, and consequently, along a separate path. For the present invention, the X-ray source is oriented relative to the object to direct all such paths towards the object. Further, a mechanism is provided to move the X-ray source relative to the object to cause each radiation beam emanating from the X-ray source to successively travel on different paths through the object. For example, the X-ray source can be slideably mounted on a circular track that extends around the object.
A detector array is positioned on the opposite side of the object to interpose the object between the X-ray source and the detector array. Preferably, the detector array includes a plurality of detectors, one detector for each beam that emanates from the X-ray source/collimator assembly.
Further, a mechanism can be provided to move the detector array as the X-ray source is moved. Specifically, the detector array can be moved to allow each detector to track a single X-ray beam, as that X-ray beam travels on successive paths through the object. In response to the receipt of an X-ray beam, the detector produces an electrical signal that is proportional to the intensity of the radiation received.
An important aspect of the present invention is that the X-ray radiation is filtered between the X-ray source and the detectors. Specifically, for each X-ray beam on each path, the beam is successively filtered four times, each time with a different filter. Each time the beam is moved to a new path, the beam is once again filtered four times. Each time the beam is successively filtered four times, four different electrical signals are produced by a detector.
For the present invention, these four electrical signals can be manipulated by a processor to produce an image signal for the path. Once an image signal is established for each desired path, conventional tomography techniques can be used to combine all the image signals (one image. signal for each path) into a composite image that reveals the internal features of the object.
For the present invention, a filter set having four different filters is used to successively filter each beam on each path. In accordance with the present invention, a unique filter set is designed for use with a specific contrast agent that is prescribed for introduction into the object. Specifically, the chemical constituents and thickness of each filter are determined with reference to the specific contrast agent that is being used. For a contrast agent with a chemical element having a KEDGE, CONTRAST AGENT, a filter set is used having a first filter with a chemical element having a KEDGE that is greater than KEDGE, CONTRAST AGENT, a second filter with a chemical element having a KEDGE that is greater than or equal to KEDGE, CONTRAST AGENT, a third filter with a chemical element having a KEDGE that is less than or equal to KEDGE, CONTRAST AGENT, and a fourth filter with a chemical element having a KEDGE that is less than KEDGE, CONTRAST AGENT.
A mechanism is provided to successively interpose each filter of the filter set between the X-ray source and the object to thereby allow the successive filtration of the beams emanating from the source. For example, the filter set can be mounted on either an oscillating frame or a rotating wheel. For the embodiment with the wheel, all four filters are mounted on the wheel, and a motor is provided to rotate the wheel about the wheel""s axis. The wheel and motor can be attached to the X-ray source to allow the wheel, filters and motor to travel with the X-ray source/collimator assembly as the assembly moves with respect to the object.
In operation, first the contrast agent is introduced into the object, and the object is placed between the X-ray source and the detector array. Next, the X-ray source is located at a first position and activated to produce one or more beams travelling through the object on a first set of paths (one path for each beam). Next, the wheel containing the filters is rotated to successively interpose each of the four filters between the X-ray source/collimator assembly and the object to filter each of the beams with each of the four filters. This results in the production of four intensity-proportional signals by a detector for each beam.
For the present invention, these four signals can be manipulated either on-line or off-line by a processor to produce an image signal for the path. Specifically, the processor subtracts the digital signal produced by the detector with the second filter interposed along the path from the digital signal produced by the detector with the first filter interposed along the path. The result of this is a first intermediary difference signal. This first intermediary difference signal simulates the image signal that would be obtained if a quasi-monochromatic beam having an average energy slightly below KEDGE, CONTRAST AGENT were to be passed through the object along the path. Similarly, the processor subtracts the digital signal produced by the detector with the fourth filter interposed along the path from the digital signal produced by the detector with the third filter interposed along the path. The result of this is a second intermediary difference signal. This second intermediary difference signal simulates the image signal that would be obtained if a quasi-monochromatic beam having an average energy slightly above KEDGE, CONTRAST AGENT were to be passed through the object along the path. Next, the processor is programmed to subtract the second intermediary difference signal from the first intermediary difference signal to produce the final image signal for the path.
Once image signals are obtained for the first set of paths, the X-ray source/collimator assembly can be moved to a second position to cause the beams emanating from the assembly to travel along a new set of paths. While the X-ray source/collimator assembly is at the second position, the wheel is again rotated to successively interpose each of the four filters between the X-ray source/collimator assembly and the object to again filter each of the beams with each of the four filters. Again, four intensity-proportional signals are produced by a detector for each beam. For the present invention, these four signals can be manipulated by a processor as described above to produce an image signal for each new path. This process of moving the X-ray source/collimator assembly and producing an image signal for each new path can be repeated as desired. Further, it is to be appreciated that the X-ray source/collimator assembly can be moved continuously around the object. When this technique is used, the wheel containing the filters can be rotated continuously as the X-ray source moves. By rotating the wheel very rapidly, each beam can be filtered four times before significant movement of the beam occurs. Thus, in effect, each beam remains on a single path while the successive filtration takes place. Once an image signal is produced for all paths of interest, conventional tomography techniques can be used to combine all the image signals (one image signal for each path) into a composite image that shows the internal features of the object.